It has been reported clinically that edrophonium (EDR) has a more potent antagonistic effect on the fade of twitch tension and a less potent antagonistic effect on the depressed single twitch force induced by muscle relaxants than does neostigmine (NST). However, the details of the mechanism have not yet been clarified. In this experiment, to compare the antagonistic effect of EDR and NST on the neuromuscular blocking effect of dTc, the recovery from dTc induced neuromuscular blocking effect on both the postjunctional membrane and the motor nerve terminal with EDR or NST was studied using intracellular recording techniques in rat phrenic nerve-hemidiaphragm preparations. The preparation was constantly perfused with physiological saline solution at room temperature. To determine the ACh sensitivity of the postjunctional membrane, ACh potentials were elicited and recorded. To determine the quantum release of ACh from the motor nerve terminal and its rundown, endplate potentials (EPPs) were elicited at 2Hz and recorded. ACh sensitivity was increased with EDR and NST in a dose dependent manner. The extent of increased ACh sensitivity with EDR 10(-6)M was almost equal to that with NST 10(-7)M. In this experiment, these concentrations were used to compare the antagonistic effect of EDR and NST on the effect of dTc 10(-6)M. The depressed ACh sensitivity with dTc was reversed with EDR and NST. The extent of restoration with NST was about three times that with EDR. The quantum release of ACh was increased and rundown was depressed with EDR, but NST had no effect. After pretreatment with dTc, neither EDR nor NST increased the decreased quantum release of ACh nor reversed the enhanced rundown induced by dTc. But under the same condition, both EDR and NST reversed the ACh sensitivity depressed with dTc. These results demonstrate that neither EDR nor NST were able to affect the decreased quantum release of ACh nor the enhanced rundown induced by dTc. The results also suggest that the reversal of fade of twitch tension with EDR and NST does not always result from recovery of rundown of EPP amplitude.